1. Field of the Invention
The present disclosure relates to a semiconductor photodetector, and in particular relates to a semiconductor photodetector which detects very weak light.
2. Description of the Related Art
In recent years, in various fields such as medical services, biotechnology, and radiation measurement, there is required a photodetector which accurately measures light as weak as a single photon. Currently, a photomultiplier tube (PMT) is widely used for detecting weak light. However, the size of a PMT, which is a vacuum tube device, is about 10 mm×10 mm at smallest, and therefore it is difficult to increase the number of pixels. In addition, when a PMT is used to perform two-dimensional imaging, it is necessary to scan an object in an X-Y plane to gather information of every point of the object and then to form an image. Therefore, it is difficult to capture an image of the object in real time. In view of the above issues, in order to realize both of multiple pixels and high speed in a photodetector for detecting very weak light, a solid state device of the photodetector is required.
As an example of photodetectors for detecting very weak light, there is proposed a photon-counting type photodetector, in which an avalanche photo diode (APD) is used. This photodetector counts photons entering an APD and transmits the result of the counting as a signal of a digital value to the outside of the pixel.
For example, as described in PTL 1, a photon-counting type photodetector has a structure with a plurality of APDs arranged in a matrix, and each APD is connected to a load resistor and is applied with a high voltage slightly lower than a breakdown voltage or not lower than the breakdown voltage. In the photodetector, a pulse signal is generated in response to one photon entering the APD, based on an operation principle to be described below, and the pulse signal increments a count value of a counter by one. By this operation, the incident photons are counted.
In the following, an operation principle of the APD will be briefly described.
A photon entering the APD generates an electron-hole pair. One or both of the generated electron and hole are accelerated by an electric field in the APD generated depending on a voltage applied to the both ends of the APD, and collide with a crystal lattice to generate another electron-hole pair. This phenomenon is referred to as “impact ionization”. This impact ionization is repeated, and the charges are thus multiplied.
The APD has two operation modes, i.e., a linear mode and a Geiger mode.
The linear mode is an operation mode in which the APD is applied with a voltage slightly lower than the breakdown voltage of the APD so that only one of the electron and the hole causes impact ionization. In the linear mode, impact ionization occurs a limited number of times, and therefore the output current is proportional to the number of the incident phones.
The Geiger mode is an operation mode in which the APD is applied with a voltage not lower than the breakdown voltage of the APD so that both of the electron and the hole cause impact ionization. In the Geiger mode, both of the electron and the hole repeat impact ionization in an avalanche manner, and the output current thus increases rapidly. Therefore, the device is usually used with a load resistor series-connected to the APD so that the device is not destroyed. In this case, if a large current flows through the load resistor, a voltage is generated on the both ends of the load resistor, and the voltage applied to the both ends of the APD is reduced by the generated voltage. When the voltage on the both ends of the APD becomes greatly lower than the breakdown voltage, the impact ionization stops, and the output current decreases rapidly. Thus, in the Geiger mode, the output current is a pulse signal, and the value of the current output from the APD is not proportional to the incident photon.
The photodetector described in PTL 1 uses an APD in the Geiger mode.